Nitrogen and Energy Budgets of Production Ewes on Summer Range in Southwestern Utah

Nitrogen (N) and energy budgets for free-roaming ewes were quantified on mountain summer range in southwestern Utah in 1986 and 1987. Diet quality (%N), in vitro organic matter digestibility (IVOMD), and energy were estimated with the use of five esophageally fistulated ewes. Excretion rates of N and energy to feces and urine were measured by total collection from five nonfistualted ewes. Total intake N(gN/(MBW*d-1) tracked intraseasonal changes of dietary N percent but was apparently buffered by fluctuations in dry matter intake. In each year, total urine N was closely related to dietary N concentration, r2=0.97 (1986) and r2=0.89 (1987). Total N excreted via feces varied as a function of dry matter output rather than fecal N concentration. Consumption of gross energy (GE) paralleled dry matter intake in both years of the study, r2=.99. Fecal energy excretion (kcal/(MBW*d-1) was associated with dry matter output while urine energy varied with little amplitude across both grazing seasons. In both years, absolute values for metabolizable energy (ME) and nitrogen balance indicated that the sheep were in a positive nutritional state throughout the grazing season. ME levels calculated by difference were consistently higher than when determined by DE X 0.82, since the gaseous products of fermentation were not accounted for in difference determinations. Ratios of estimated intake versus maintenance levels of nitrogen and energy were calculated for each grazing season under study. The data suggest that in 1986, N was relatively more limiting in early summer, while energy was relatively more limiting at the end of the season. Digestible organic matter intake appeared to be below maintenance requirements in September, 1986. In 1987, relative amounts of both nutrients paralleled each other throughout the grazing season and were above maintenance, with the exception of the September trial when both nitrogen and energy were below calculated maintenance levels